The rate of reduction of iron oxides dissolved in calcium-silicate slags by iron-carbon melts is an important factor in iron-making and steelmaking processes in general and, in particular, in in-bath smelting steelmaking processes. Several studies, for instance, have investigated how Fe-C metal alloy droplets affect the reduction rates of FeO in steelmaking processes, where the overall reaction is: EQU FeO (slag)+C (in metal)=Fe(1)+CO(g)
The reduction rates of FeO dissolved in CaO--SiO.sub.2 --Al.sub.2 O.sub.3 slags by Fe-C metal alloy droplets have been experimentally determined, e.g. Sawada, Y., M. S. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1990. In the study, Sawada found that there were two distinct regimes of reaction rates--one very fast and one very slow. The faster rate was identified as occurring during the initial 60 to 300 seconds of the reaction, depending on the slag composition and temperature. The faster reaction was found to be zeroeth order with respect to carbon in the Fe-C metal alloy droplet. The reaction rate in the second stage was very slow and the reaction virtually stopped when the carbon in the Fe-C metal alloy droplet was reduced to a particular level (commonly approximately 2 wt %). As a result, the reduction reaction fails to go to completion and leaves a slag containing FeO and Fe-C which creates a relatively high carbon intermediate.
The virtual stoppage of the reaction is believed to be due to the inability of electrons (e) (accumulated in the slag at the slag/Fe-C metal alloy interface in accordance with the reaction: O.sup.2- .fwdarw.O+2.sub.e), to migrate across the slag and be consumed by Fe.sup.2+ and Fe.sup.3+ ions in the reactions: Fe.sup.2+ +2.sub.e .fwdarw.Fe, and 2 Fe.sup.3+ +2.sub.e .fwdarw.2Fe.sup.2+. These cathodic reactions are believed to control the rate of the decarburization reaction. When the reaction Fe.sup.2+ +2.sub.e .fwdarw.Fe and/or Fe.sup.3+ 2.sub.e .fwdarw.2Fe.sup.2+ ceases, the oxidation of O.sup.2- per the reaction O.sup.2- O+2.sub.e is blocked. As a result, CO gas stops being formed according to the reaction: C+O.fwdarw.CO, Fe-C metal alloy droplets stop being decarburized, and the overall reaction (FeO (slag) +C (in Fe-C metal alloy)=Fe(l)+CO(g)) stops.
Consequently, conventional iron and steelmaking processes have required additional processing to convert the high carbon intermediate to metal. For instance, oxygen is typically blown through the molten slag. Such additional processing is, however, both time-consuming and adds to the expense of the process. Moreover, the additional processing produces excessively high oxygen levels in the slag and the metal, and also increases the volume of the slag by oxidizing the iron.
Conventional ironmaking and steelmaking technologies require that iron and steel be produced in two separate reactors. In iron-making processes, an ironmaking reactor (which has a reducing atmosphere) typically produces a solution of iron and carbon. In steelmaking processes, a steelmaking reactor (which typically has an oxidizing atmosphere) removes carbon from the iron and thereby produces steel. The need to use two (2) separate processes to make iron and then steel has several drawbacks. In addition to having the disadvantages of conventional steelmaking processes discussed above, the need to make iron and steel in two separate reactors increases the time and cost of production. Moreover, the making of iron and steel in two separate reactors often results in the loss of alloying elements as well as decreases in overall yield.
U.S. Pat. No. 5,314,524 discloses a process for improving the rate of metal production and FeO utilization in an iron and steel-making process wherein Fe-C metal alloy droplets are submerged in an FeO-containing slag and in which a charge build-up situated between the slag and the Fe-C metal alloy droplets (slag/Fe-C metal alloy interface) is discharged, preferably by means of an inert metallic conductor. The inert metallic conductor enables the accumulated electrons creating the charge build-up to migrate across the slag and be consumed by Fe.sup.2+ and Fe.sup.3+ in the slag according to the rate determining reactions: Fe.sup.2+ +2.sub.e .fwdarw.Fe and 2Fe.sup.3+ +2.sub.e .fwdarw.2Fe.sup.2+. Alternatively or in combination, the '524 patent discloses adding a transition-metal oxide to the FeO slag to increase the concentration of variable valence cations which consume the accumulated electrons at the slag/Fe-C metal alloy interface and to facilitate the formation of CO by the reaction C+O.fwdarw.CO. The '524 process decarburizes metallic Fe-C metal alloy droplets submerged in an FeO-containing slag to less than about 0.05 wt % and eliminates the need to blow oxygen gas into the molten slag/metal system. The '524 process is carried out in a reactor in an inert atmosphere, which may be formed by introducing a suitable gas, i.e. argon, into the reactor. Typically, such an inert atmosphere has an oxygen partial pressure of less than about 0.001 atm.
Although the process of '524 is advantageous, the Fe.sup.2+ +2.sub.e .fwdarw.Fe and 2Fe.sup.3+ 2.sub.e .fwdarw.2Fe.sup.2+ reactions broad-scale application. Consequently, it is highly desirable to enhance the rate of removal of accumulated electrons (charge buildup) at the slag/Fe-C metal alloy interface(s) still further. Moreover, the need for a slag in '524 to contain large amounts of transition-metal oxides is expensive and not practical. It would thus be desirable to reduce these amounts substantially.
Accordingly, it is an object of the present invention to increase the rate of decarburization and thereby the rate of the reduction of iron oxides taking place in a FeO-containing slag.
Another object is to substantially reduce the amount of FeO and transition-metal oxides utilized.
A still further object is to produce iron and steel in a single reactor in a single continuous process.